The present invention relates to a rear-wheel steering apparatus having an electric motor to perform phase inversion control of a steering angle ratio of rear wheels in case of turning. More particularly, it relates to assurance of safety in case of failure in a control system of rear-wheel steering.
A steering angle ratio .theta..sub.S is defined by a ratio (.theta..sub.R /.theta..sub.F) of a steering angle .theta..sub.F of front wheel with respect to a steering angle .theta..sub.R of rear wheel.
A rear-wheel steering apparatus for vehicles is generally arranged so as to control the rear wheels so that the steering angle of the rear wheels becomes a predetermined value in accordance with the steering angle of front wheels. This control is performed by a ratio control in accordance with a predetermined steering-angle-ratio set corresponding to vehicle speed. The ratio control is called a speed sensitive type rear-wheel steering apparatus.
In such apparatus, in order to secure a direction stability of vehicle, it is generally controlled so that the steering angle ratio becomes a positive value in an intermediate to high speed region. That is, it is controlled so that directions of steering angles of the front and rear wheels are in an in-phase state side when the steering handle begins to turn. However, since this is performed as a ratio control, the rear wheels are steered (turned) to an in-phase state. Accordingly, sufficient head rotatability cannot be obtained.
To solve the above problem, a phase inversion type rear-wheel steering apparatus is suggested in Japanese Patent Publication Kokai Nos. 57-44568 and 2-249765. The apparatus is capable of detecting a yaw rate signal by a yaw movement of vehicle, and turn the rear wheels to a phase which is opposite to that of front wheels immediately after the front wheels start to be steered, and then to an in-phase state in accordance with an occurrence of the yaw rate signal. This is so called a "phase inversion control by yaw rate feed back". Head rotatability and direction stability are balanced by this control.
Accordingly, in the phase inversion control by yaw rate feed back, it is extremely important that a steering angle ratio is changed from the opposite phase to in-phase and is performed in a short time immediately after the front wheels have begun to turn. In the phase inversion type steering angle apparatus, it is suggested that a high-speed motor be used in order to change the steering angle ratio from the opposite phase to in-phase in a short time.
On the other hand, in a rear-wheel steering apparatus which is arranged so that rear wheels are actually driven by a hydraulic power cylinder and the steering angle ratio is controlled by an electric motor such as a step motor, a spring is provided in the cylinder so as to force the rear wheels at a neutral position for fail-safe operation, if the oil pressure is not adequate and a release circuit is also provided on a hydraulic circuit. If it is detected that the step motor fails to open, the oil pressure is let out from the cylinder through the release circuit and the spring forces the rear wheels to return to the neutral position (so called "2WS position") for extra safety in case of failure.
However, when the high speed motor is adopted to the conventional rear-wheel steering apparatus which performs the phase inversion control by controlling a steering angle ratio, there appears a specific problem which did not arise in the past. This problem is discussed with reference to FIG. 1.
In the rear-wheel steering apparatus for a vehicle, since a high degree of oil pressure is supplied to the hydraulic power cylinder which is a driving source for steering the rear wheels, if the release circuit releases the oil pressure in a short time, a sudden change is caused in the steering angle of the rear wheels and it is too risky for the vehicle even for fail-safe operation. Therefore, in the prior art, an orifice is provided on a release line of the release circuit and the oil pressure is slowly reduced so that the movement of vehicle gradually changes.
FIG. 1 is a diagram to explain the operation of the speed sensitive type steering apparatus having the above-described power cylinder. In FIG. 1, when a failure is detected in the apparatus and the oil pressure is released at the time t.sub.1, the oil pressure in the power cylinder gradually decreases as shown by curve b. The rear wheels reach the neutral position at the time t.sub.3 where the oil pressure in the power cylinder and a repellent force of the spring in the cylinder are balanced. In a speed sensitive type rear-wheel steering apparatus which does not perform the phase inversion control, even if a failure occurs in a process where the phase of the steering angle ratio is changed from in-phase to opposite phase, the problem will not arise since the change of the steering angle ratio is gradual and the oil pressure has been released by the time (t.sub.3 of FIG. 1) prior to the time t.sub.4 where the phase comes to the opposite phase where rotatability is emphasized.
As described above, in the rear-wheel steering apparatus which performs the phase inversion control by controlling the steering angle ratio, the phase inversion is performed at extremely high speed in comparison with the speed sensitive type apparatus. Therefore, a case as shown in the graph d of FIG. 1 is assumed in which a failure has occurred when a steering angle ratio is going to change due to a driver's steering operation when the steering angle ratio is in in-phase state. If the failure is one occurring to the high speed motor for steering angle ratio control, the steering angle ratio has changed from in-phase to opposite phase while the oil pressure still remains in the power cylinder (that is, between time t.sub.1 and time t.sub.3 where the rear wheels come to the neutral state). During the period between time t.sub.1 and t.sub.3, a movement which cannot be expected by a driver may be caused to the vehicle.
This problem arises when the high speed motor is adopted in the phase inversion type steering apparatus. Furthermore, this problem will remain as long as the high speed motor means is used and the failure occurs where the oil pressure supplied to the power cylinder needs to be released regardless of the fact that the failure occurred in a control system of the motor.